Rework the Destination filesystem to make --renderStaticToDisk work

See #9626

cache/filecache/filecache_config_test.go

@@ -184,7 +184,7 @@ dir = "/"
 }
 
 func newTestConfig() config.Provider {
-	cfg := config.New()
+	cfg := config.NewWithTestDefaults()
 	cfg.Set("workingDir", filepath.FromSlash("/my/cool/hugoproject"))
 	cfg.Set("contentDir", "content")
 	cfg.Set("dataDir", "data")

cache/filecache/filecache_test.go

@@ -342,6 +342,7 @@ func newPathsSpec(t *testing.T, fs afero.Fs, configStr string) *helpers.PathSpec
 	cfg, err := config.FromConfigString(configStr, "toml")
 	c.Assert(err, qt.IsNil)
 	initConfig(fs, cfg)
+	config.SetBaseTestDefaults(cfg)
 	p, err := helpers.NewPathSpec(hugofs.NewFrom(fs, cfg), cfg, nil)
 	c.Assert(err, qt.IsNil)
 	return p